The M-209
cipher machine was used extensively by the US armed forces in the period
1943-45. Army units in England sent training messages on the M-209 which the
Germans decoded.

The USAAF used it in operational and
administrative networks.

M-209 traffic together with D/F may
have allowed the Germans to discover the concentration of US forces in the
South.

After having
a look at the report E-Bericht Nr. 3/44
der NAASt 5 (Berichtszeit 1.4-30.6.44) it is clear that the Germans were in
fact able to get order of battle intelligence on the US forces in the UK. In
pages 2-3 it says:

Activity report before the invasion

…………………………………………

1). AM1:

Focused on decoding the AM1. Ten
absolute settings were recovered, which brought the deciphering of 1,119
messages. This cipher-material, mostly composed by the U.S American
Expeditionary Corps, gave valuable insights into the location of enemy groups.

AM1 (Amerikanische
Maschine 1) was the German designation for the M-209.